Encapsulation of thermal energy storage media

ABSTRACT

In one embodiment, a method for depositing metal on a polymer surface, the method includes coating the polymer surface with a binding metal to render the polymer surface solvophillic and/or hydrophilic and depositing a further metal on the binding metal-coated polymer surface.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of co-pending U.S. application Ser. No.14/425,415, filed Mar. 3, 2015, which claims priority to 35 U.S.C. § 371national stage of PCT application PCT/US2013/075971, filed Dec. 18,2013, which claims priority to and the benefit of U.S. ProvisionalApplication No. 61/738,655, filed on Dec. 18, 2012, herein incorporatedby reference in their entirety.

NOTICE OF GOVERNMENT-SPONSORED RESEARCH

This invention was made with Government support under grant contractnumber DE-EE0003490 awarded by the U.S. Department of Energy (DOE). TheGovernment has certain rights in the invention.

BACKGROUND

Thermal energy storage is critical to the future of solar thermal power.Such storage reduces the cost of the energy and enables the energy to beavailable both during and after daylight hours. The selection ofappropriate thermal energy storage media is important to developing aneffective thermal energy storage scheme. Phase change materials (PCMs)are particularly attractive for such applications because they can storemuch more heat than sensible heat storage materials.

It would be desirable to use PCMs both in future installations as wellas existing installations that are not specifically configured for useof such materials. In view of this, it would further be desirable todevelop a way to encapsulate PCMs so that they can be used in bothfuture and existing installations.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure may be better understood with reference to thefollowing figures. Matching reference numerals designate correspondingparts throughout the figures, which are not necessarily drawn to scale.

FIG. 1 is a flow diagram of an embodiment of a method for encapsulatingthermal energy storage media, such as phase change material (PCM).

FIG. 2 is a side view of an embodiment of a pellet of thermal energystorage media that is to be encapsulated.

FIG. 3 is a side view of a further embodiment of a pellet of thermalenergy storage media having a pre-formed internal void.

FIG. 4 is a cross-sectional view of a pellet that has been coated with alayer of flexible material.

FIG. 5 is a cross-sectional view of the coated pellet of FIG. 4 afterthe pellet has been heated.

FIG. 6 is a cross-sectional view of the coated pellet of FIG. 5 after ithas been cooled.

FIG. 7 is a cross-sectional view of the coated pellet of FIG. 6 after ithas been further coated with metal.

FIG. 8 is a cross-sectional view of an alternative embodiment for athermal energy storage media capsule.

FIG. 9 is a flow diagram of an embodiment of a method for metalizing ahydrophobic polymer.

DETAILED DESCRIPTION

As described above, it would be desirable to have a way to encapsulatethermal energy storage media, such as phase change materials (PCMs).Disclosed herein are encapsulation techniques and capsules that areappropriate for this purpose. In some embodiments, a pellet of solid PCMis first coated with a layer of flexible material and the coated pelletis then heated. The PCM expands as it is heated as does the flexiblematerial. During this heating, air contained within the pores of thesolid pellet diffuses out through the pores of the flexible material.Once the temperature rises above the melting temperature of the PCM, thePCM melts and, eventually, no solid PCM remains. The coated pellet isthen enabled to cool. As the molten PCM cools it solidifies from theoutside—in so that the pellet maintains much of its increased size. Asthe molten PCM solidifies, it shrinks and air that was dissolved in themolten PCM migrates from the solidifying PCM to the remaining molten PCMin the center of the pellet. This air ultimately forms a void within thecenter of the solidified pellet into which melted PCM can later expand.Metal can then be applied to the flexible material to form a rigid PCMcapsule.

In the following disclosure, various specific embodiments are described.It is to be understood that those embodiments are exampleimplementations of the disclosed inventions and that alternativeembodiments are possible. All such embodiments are intended to fallwithin the scope of this disclosure.

Generally speaking, there are two main obstacles to the successfulencapsulation of thermal energy storage media, such as PCM. The firstobstacle is the expansion of the PCM upon melting. The second obstacleis the expansion of air comprised within pores of the PCM. The combinedexpansion of the PCM and the air creates huge pressure build up withinthe capsule, which could ultimately result its rupture.

The problem of PCM expansion can be resolved by (i) inducing andmatching the porosity in the PCM pellet with the percentage expansion ofthe molten PCM, (ii) creating a void inside the pellet that is equal toor greater than the expansion of the molten PCM, and/or (iii) providinga flexible coating over the PCM pellet that enables the expanding air toescape. Of these three solutions, the latter solution enables thefabrication of the PCM pellet with little concern over the availablevoid space for the PCM expansion. Accordingly, this enables costreduction by decreasing the number of steps involved in themanufacturing process. However, the problem of air expansion can only beresolved by having a selectively permeable coating over the pellet thatenables air, but not molten PCM, to pass.

The above-described challenges can be overcome using the disclosedencapsulation processes. One such process comprises coating a solidpellet of PCM with a flexible, selectively permeable coating, slowlyheating the coated pellet to enable the gradual diffusion of air throughpores of the coating, cooling the coated pellet to enable the formationof an internal void into which molten PCM can expand, and then furthercoating the pellet with metal to form a rigid PCM capsule.

FIG. 1 is a flow diagram of an example method for encapsulating PCM inthe manner identified above. Beginning with block 10 of FIG. 1, a pelletof PCM is formed. An example pellet 20 is illustrated in FIG. 2. As isshown in that figure, the pellet 20 can be generally spherical. Thatshape enables close packing of PCM capsules in, for example, a storagetank. The PCM can be substantially any PCM that would be desirable forthermal energy storage. In some embodiments, the PCM comprises anorganic or inorganic salt, a salt mixture, or a salt eutectic having arelatively high melting temperature. One example of a particularlysuitable salt is sodium nitrate (NaNO₃), which has a melting point ofapproximately 306° C. Table I identifies this salt, as well as variousother example PCMs that could be used.

TABLE I Example PCMs PCM (wt %) Melting Point (° C.) LiNO₃(30%) +NaNO₃(18%) + KNO₃(52%) 120 LiNO₃(33)—67KNO₃ 133LiNO₃(55.4)—4.5NaNO₃—40.1KCl 160 LiNO₃(58.1)—41.9KCl 166 NaOH(50)—50KOH171 LiNO₃(57)—43NaNO₃ 193 LiNO₃(49)—51NaNO₃ 194 LiOH(30)—70NaOH 216NaNO₃(54)—46KNO₃ 222 NaOH(20)—80NaNO₂ 230 NaOH(73)—27NaNO₂ 237 NaNO₂ 270NaNO₃ 306 KNO₃ 334 LiCl(32.90)—34.8NaCl—32.3KCl 357MgCl₂(60)—19.6NaCl—20.4KCl 383

In some embodiments, the pellet 20 has a diameter of approximately 1 to2 inches. Of course, the pellet 20 can have larger or smallerdimensions, as well as different shapes, depending upon the applicationand the desired outcome. The pellet 20 can be completely solid with nointernal voids. In other embodiments, the pellet can have a pre-formedinternal void that provides space for the PCM to expand into when itmelts. FIG. 3 shows an example of a pellet 22 that comprises an internalvoid 24. As shown in that figure, the pellet 22 can be formed from twohalves 26 that can be joined together to form the whole pellet. Severalavailable PCMs are powerful oxidizers, especially in the molten state.These PCMs have great potential for reacting with many organic andinorganic reducing agents. Therefore, the use of an organic bondingmaterial for joining the two halves 26 of the pellet 22 may not besuitable as it reacts with the PCM, which may not only cause failure ofthe coating but may also reduce the required thermophysical propertiesof the PCM used. Such problems can be avoided by joining the two halves26 of the pellet 20 by heat fusing them together. In some embodiments,this process involves blowing hot air over the joint formed by the twohalves 26 until the PCM located at the joint melts and later solidifiesin a controlled manner. Another approach involves applying molten PCMover the joint of the pellet 20, which is kept at ambient temperature.The molten PCM used for this purpose could be the same material oranother material that melts at a lower temperature.

With reference back to FIG. 1, once a pellet has been formed it can becoated with flexible material to form a coated pellet, as indicated inblock 12. In some embodiments, the flexible material is a polymericmaterial. To avoid changing the thermophysical properties of the PCM, anon-reactive polymeric material can be selected that will not react withthe PCM. Example non-reactive polymeric materials includepolytetrafluoroethylene (PTFE), fluorinated ethylene propylene (FEP),perfluoroalkoxy (PFA), and mixtures thereof. As described below, a layerof metal can be formed over the coated pellet to provide greaterstructural integrity to the PCM capsule and to improve its performanceand longevity, particularly at temperatures higher than the meltingpoint of the flexible material. As is further described below, such alayer can be formed using electroless and electrolytic processes thatinvolve the application of plating solutions that may contain additives,such as organic substances, that can react with the PCM. Unfortunately,it is possible for this plating solution to permeate the non-reactivepolymeric material applied to the PCM pellet and therefore reach thePCM. In addition, water from the plating solutions may reach the PCM andcreate expansion problems that could later rupture the capsule. In suchsituations, it is desirable to apply further polymeric material that isimpermeable to such solutions. Examples of impermeable polymericmaterials include polyimide, polyvinylidene fluoride (PVDF), andmixtures thereof. While solvents used to apply these materials can reactwith PCMs, the first polymer layer (e.g., of PTFE, FEP, PFA, or acombination thereof), prevents the second polymer layer from reachingthe PCM. In view of the above discussion, in some embodiments, coatingthe pellet with a flexible material comprises first applying anon-reactive (to the PCM) polymeric material to the pellet and thenapplying an impermeable (to the plating solution) polymeric materialover the non-reactive polymeric material. Irrespective of the polymericmaterial or materials that are used, the materials are selectivelypermeable, meaning that they enable air to pass but do not enable moltenPCM to pass.

FIG. 4 illustrates the pellet 20 (which is assumed for this example notto comprise an internal void) after it has been coated with a layer 30of flexible material. As described above, the layer 20 can comprisemultiple individual layers of polymeric material, which can bedifferent. In some embodiments, the layer or layers can be formed bypowder coating with polymeric particles and then applying pressure tofuse the polymeric particles together to form a uniform polymeric layer.In other embodiments, strips of polymer film are wrapped around the PCMpellet 20 and the wrapped pellet is pressed to form a monolithic layerof polymer over the pellet. When multiple polymeric layers are used,strips of a first polymer film can be applied to the pellet 20, stripsof a second polymer film can be applied to the first polymer film, andthe coated pellet can be pressed and heated to fuse the two differentpolymers together to form a uniform layer 30 of flexible material. Instill other embodiments, one or more types of polymer film are formedinto a desired (e.g., spherical) shape and molten PCM is poured into it,followed by pressing to form the desired shape of the coated pellet.Regardless of the technique used to form the layer 30, the layer can,for example, be approximately 0.1 to 2 mm thick.

With reference next to block 14 of FIG. 1, the coated pellet is thenheated to a temperature beyond the melting point of the PCM. In someembodiments, the coated pellet is heated to a temperature that is onlymoderately higher than the PCM melting point. For example, if the PCMhas a melting point of approximately 306° C., the pellet can be heatedto approximately 316° C. As the coated pellet is heated and the PCMmelts, the PCM and the air that was contained within the pores of thesolid PCM expand. As a result, as shown in FIG. 5, the pellet 20 and itslayer 30 of flexible material likewise expand. Because the layer 30 isselectively permeable, the air within the PCM can diffuse out throughthe layer 30 of flexible material under the increased pressure withinthe pellet 20. In some embodiments, the diameter of the coated pelletcan increase by up to approximately 20% once all of the PCM has melted.It is noted that this heating may also cause blending of two or morepolymeric materials used to form the flexible layer 30 of material.

Referring next to block 16 of FIG. 1, the coated pellet is then cooledto enable the PCM to solidify. During the cooling process, the PCMsolidifies from the outside—in so that much of the increased dimensionof the pellet 28 is maintained. As the remainder of the PCM solidifies,air that was dissolved in the molten PCM migrates to the center of thepellet 28 and ultimately forms an internal void 32, as shown in FIG. 6,which provides space for expansion of the PCM when the pellet isreheated and the PCM again melts.

Once the entire mass of PCM has solidified, metal can be deposited onthe coated pellet to form a rigid capsule, as indicated in block 18 ofFIG. 1. FIG. 7 shows the pellet 20 including a layer 34 of metal. Insome embodiments, the metal layer 34 can comprise multiple individuallayers of different metals. In some cases, binding metal is firstapplied to the coated pellet to render the polymer layer 30 solvophillicand/or hydrophilic and enable another metal to adhere to the coatedpellet. For example, small particles (e.g., 20 to 30 μm in diameter) ofnickel, palladium, aluminum, copper, or alloys thereof can be firstapplied to the coated pellet using a powder coating technique such asrubbing, jar milling, or rolling the coated pellet in the particles.Once deposited, the particles act as binding agent for subsequent metalthat is deposited. Notably, the binding metal can, in some embodiments,be deposited on the coated pellet prior to the heating step identifiedin relation to block 14.

After the binding metal particles have been deposited, one or morefurther layers of metal can be deposited. In some embodiments, thefurther layers of metal can comprise copper, nickel, tin, palladium,cobalt, silver, zinc or alloys thereof and can be deposited using anelectroless deposition technique.

In the electroless deposition process, a catalyst is applied to theobject to be plated to catalyze and initiate the deposition of metal.The deposition of catalyst can be accomplished by a process that can bebroadly categorized as a water-based process or an organic solvent-basedprocess. Water-based processes often employ colloidal palladium. Inorganic solvent-based processes, a precursor of palladium, such aspalladium acetate or one or more other organic ligand-based palladiumcompounds, is used. In this process, the palladium precursor isdecomposed either thermally or by the application of some reducing agentto obtain zero valent palladium.

In some embodiments, an organic solvent solution is used to deposit acatalyst precursor on the polymeric material. For example, acetone canbe used to deposit palladium acetate on the material. Once the solutionhas been applied, it can be heated to decompose the palladium acetate.By way of example, the solution can be heated to a temperature ofapproximately 250 to 300° C. In alternative embodiments, however, areducing agent, such as sodium borohydride, hydrazine hydrate, lithiumaluminum hydride, or the like, could be added to the solution todecompose the palladium acetate at room temperature.

Once the coated pellet has been rendered electrically conductive throughthe electroless deposition of metal, electroplating can then be used toapply further metal. Accordingly, the metal layer 34 can, in someembodiments, be formed using a three-step process in which a bindingmetal is first deposited, further metal is then deposited usingelectroless plating, and then additional metal is next deposited usingelectroplating. Irrespective of the metals or deposition techniquesused, the completed metal layer 34 can be approximately less than amicron to greater than 2.0 mm thick.

It is noted that a metal layer 34 need not be formed in all situations.For example, if a metal layer is not needed for structural integrity,the PCM capsule may only comprise the polymer-coated PCM pellet. It isfurther noted that multiple alternate polymer and metal layers can beformed, if desired. For example, a PCM capsule can be formed thatincludes a first polymer layer covered by a first metal layer, which iscovered by a second polymer layer, which is, in turn, covered by asecond metal layer. FIG. 8 illustrates such a capsule 40, which includesa PCM pellet 42, a first polymer layer 44, a first metal layer 46, asecond polymer layer 48, and a second metal layer 50.

Described below are multiple examples of pellet encapsulation that wereperformed to fabricate PCM capsules.

Example 1

Sodium nitrate pellets having a 27 mm diameter were formed with a 30%void space inside. The pellets were formed from two halves that werefused together by blowing hot air (by using hot air nozzle) over thejoint formed between the halves. No foreign material was used for fusingthe pellets. The fused pellets were then wrapped with PTFE film (1 g or2 g). PTFE was chosen because there was no evident change in thedecomposition profile of the PTFE in contact with sodium nitrate (TGAanalysis) up to 530° C. The PTFE-wrapped pellets were then heated at325° C. for a few minutes and pressed to fuse the multiple layers of thePTFE tape into monolithic layers. This prevented the seepage of aqueousand non-aqueous solution from the metallization step into the pellets.Without the formation of monolithic layers, the rejection rate duringthe metallization step was large. However, the thermal pressingdrastically reduced the rejection rate.

Next, the monolithic PTFE-wrapped pellets were metallized usingelectroless and electroplating processes. The metals used were nickel,copper, zinc or zinc-iron, or zinc-nickel alloy. The metallization wasachieved by applying multiple layers of metal. The first step involvedthe deposition of a very thin layer of nickel (1-10 μm) over the PTFE.The layer of metal was intentionally left porous. The metal-coatedpellets were then heated to 316 or 326° C. (10-20° C. above the meltingpoint of sodium nitrate) for 80 minutes. During this step, the sodiumnitrate melted and migrated into the voids, and the air that was trappedin the voids escaped through the porous coating layers.

When the sodium nitrate begins to solidify it contracts. The negativepressure of that contraction pulls the coating inward and thereforewrinkles were seen in the outer layers of the pellets. Diffusion of airback through the coatings and solidified salt was hindered by the metaland the outer solid salt layers. Another layer of nickel was then builtover the top of the thermal-cycled pellets. The thermal heat cycle (326°C.) was repeated and the pellets were plated again. This process wasrepeated till a non-permeable (i.e., non-permeable to air as well asmolten salt) metal layer was obtained. This metal layer was formedthrough four metallization steps resulting in approximately 100-200 μmof metal covering the PTFE-coated salt pellet. The salt capsule was thentested and has gone through more than 1,000 thermal cycles withoutfailure.

Example 2

Sodium nitrate pellets were formed as described in Example 1. A layer ofPTFE was formed over the pellet followed by a layer of FEP and then afurther layer of PTFE to form a flexible layer having three individualpolymer layers. The coated pellet was then heated to 326° C. for onehour. During this time, the FEP melted and blended with the PTFE to forma uniform PTFE-FEP composite layer over the pellet. The coated pelletswere then metallized and tested as described in Example 1.

Example 3

Salt pellets were coated with PTFE and then coated with polyimide(PI-84) resin. The resin was cured at 290° C. for one hour. The coatedpellets were subjected to a first thermal cycle at 316° C. and thenmetallized with a combination of nickel and copper. The pellets weretested at 350° C. for nine cycles and then at 326° C. for 30 cycles.

Example 4

A PTFE-coated pellet having an internal void was coated with a carbonfiber and polyimide composite material and cured at 290° C. for onehour. The coated pellet was tested at 350° C.

Example 5

A PTFE-coated pellet having an internal void was subjected to heating at350° C. for 80 minutes. After the first thermal cycle, the PTFE-coatedpellet was metalized as described in Example 1 to form a capsule and wassubjected to thermal cycles. The capsule underwent 300 thermal cycles.

Example 6

A PTFE-coated pellet having no internal void was subjected to heating at326° C. for 80 minutes. The size of the pellet increased because of theexpansion of the salt. After cooling, the PTFE was metalized asdescribed in Example 1 and was subjected to greater than 2,000 thermalcycles.

Example 7

A PTFE-coated pellet having no internal void was subjected to heating at326° C. then metalized as described in Example 1. A further layer ofPTFE was wrapped over the metal layer. The outer PTFE layer was thenmetalized by electroless and electroplating processes. Accordingly, adouble layer of polymer and metal was formed over the salt pellet. Theamount of coating material was the same as that of a single-layeredpellet. The pellet was tested at 350° C. for 80 minutes. The pelletssurvived about 450 thermal cycles. The average number of cycles survivedat 350° C. by double-layered pellets was greater than the single-layeredpellets. It is pertinent to mention that thermal cycling at 350° C. was22° C. above the melting point of the PTFE. The double metal layersandwiches a PTFE layer to prevent physical deformation of the PTFEabove its melting point and thus lead to better performance.

Example 8

A LiCl—NaCl—KCl eutectic hemispherical pellet having no internal voidwas fabricated. This eutectic material is highly hygroscopic, whichaffects its thermophysical properties. PTFE tape (1 g or 2 g) waswrapped over the pellet and pressed in an automatic hydraulic press byapplying 7,000 lbs. of force to form a transparent, uniform film of PTFEover the eutectic material. A thin layer of nickel was deposited overthe PTFE by electroless process as described above. The metalized pelletwas then heated to 380° C. for one hour. During this step, the eutecticmaterial melted and expanded, and the moisture in the material escaped,leaving a pure eutectic material having optimum thermophysicalproperties. The pellet was then electroplated with nickel to 200 μm ofthickness. The resulting capsule was then tested at 380° C. for 80minutes.

Example 9

A salt pellet of sodium nitrate(54)-46potassium nitrate eutectic, whichmelts at 222° C., was coated with FEP and subjected to thermal cyclingat 240° C. No metal was applied to the coated pellet.

Example 10

A salt pellet of lithium nitrate(33)-67potassium nitrate eutectic, whichmelts at 133° C., was coated with FEP and subjected to thermal cyclingat 170° C. with water, air, or oil as heat transfer fluids. No metal wasapplied to the coated pellet.

As can be appreciated from the foregoing discussion, the disclosedthermal energy storage media capsules enable solar energy to becollected and stored during the day and to be used at night and at othernon-solar times. Moreover, they provide the thermal energy storage atlow cost. The capsules may reduce the cost of thermal energy storage forsolar thermal power from about $50/kWh_(th) to less than $15/kWh_(th).The encapsulated thermal energy storage is also useful for nuclear powerand many industrial applications.

While the foregoing discussion has focused on the formation of thermalenergy storage media capsules, it is noted that processes used to formthe capsules can be used in other contexts. For example, the disclosedmetallization steps can be used to apply metal to hydrophobic polymers.FIG. 9 illustrates an example of such a method. Beginning with block 60,a hydrophobic polymer object is obtained. The polymer can comprisesubstantially any hydrophobic polymer. Examples include PTFE, FEP, PFA,polyimide, PVDF, and mixtures thereof.

Next, as indicated in block 62, binding metal is applied to the objectto render it solvophillic and/or hydrophilic. For example, smallparticles (e.g., 20 to 30 μm in diameter) of nickel, palladium,aluminum, copper, and alloys thereof can be applied to the object to actas binding agent for subsequent metal that is to be deposited. In someembodiments, the particles can be applied using powder coatingtechniques, such as rubbing, jar milling, or rolling.

Once the binding metal has been applied, metal can be deposited on theobject to form a metallized object, as indicated in block 64. In someembodiments, the further layers of metal can comprise copper, nickel,tin, palladium, cobalt, silver, zinc, or alloys thereof and can bedeposited using an electroless deposition technique followed byelectroplating once the object has been rendered electrically conductivethrough the electroless deposition of metal. Accordingly, as with themetallization of coated pellets, the hydrophobic polymer can bemetallized using a three-step process in which a binding metal is firstdeposited, further metal is then deposited using electroless plating,and then additional metal is next deposited using electroplating.Irrespective of the metals or deposition techniques used, the completedmetal layer can be approximately less than a micron to greater than 2 mmthick.

The invention claimed is:
 1. A method for depositing metal on a polymersurface, the method comprising: obtaining a hydrophobic polymer having asurface; powder coating the surface of the hydrophobic polymer withsmall particles of a binding metal to render the polymer surfacesolvophillic and/or hydrophilic; electrolessly depositing a first layerof plating metal on the binding metal-coated polymer surface to renderthe surface electrically conductive; and directly depositing viaelectroplating a second layer of plating metal on the first layer ofplating metal such that no other material is provided between the firstand second layers of plating metal.
 2. The method of claim 1, whereinthe hydrophobic polymer comprises polytetrafluoroethylene (PTFE),fluorinated ethylene propylene (FEP), perfluoroalkoxy (PFA), polyimide,polyvinylidene fluoride (PVDF), or mixtures thereof.
 3. The method ofclaim 1, wherein powder coating comprises applying nickel, palladium,aluminum, copper, or alloys thereof to the surface of the hydrophobicpolymer.
 4. The method of claim 1, wherein the particles areapproximately 2 to 30 microns in diameter.
 5. The method of claim 1,wherein powder coating comprises one of rubbing, jar milling, or rollingthe particles onto the surface of the hydrophobic polymer.
 6. The methodof claim 1, wherein the first layer of plating metal comprises copper,nickel, tin, palladium, cobalt, silver, zinc or alloys thereof.
 7. Themethod of claim 1, wherein the second layer of plating metal comprisescopper, nickel, tin, palladium, cobalt, silver, zinc or alloys thereof.8. The method of claim 1, wherein electrolessly depositing comprisesfirst applying a catalyst to the binding metal-coated polymer surface.9. The method of claim 8, wherein applying a catalyst comprises applyinga catalyst precursor using an organic solvent solution.
 10. The methodof claim 8, wherein electrolessly depositing comprises electrolesslydepositing the first layer of plating metal using a water-based process.11. The method of claim 8, wherein electrolessly depositing compriseselectrolessly depositing the first layer of plating metal using anorganic solvent-based process.
 12. The method of claim 11, wherein theorganic solvent-based process comprises decomposing a metal precursoreither thermally or by application of a reducing agent.